1. Field of the Invention
The present invention relates to a process for preparing acrolein by heterogeneously catalyzed partial gas phase oxidation, by conducting a starting reaction gas mixture which comprises propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar O2:C3H6 ratio of ≧1, and also carbon dioxide and saturated hydrocarbons together in a total amount of at most 15 mol %, at elevated temperature and at an hourly space velocity on the fixed catalyst bed of propene contained in the starting reaction gas mixture of ≧120 l (STP)/l·h through a fixed catalyst bed whose catalysts are annular unsupported catalysts whose active composition is at least one multimetal oxide of the general formula I,Mo12WaCobFecBidSieKfOn  (I)where    a=from ≧1 to ≦3,    b=from ≧3 to ≦8,    c=from ≧1 to ≦4,    d=from ≧0.5 to ≦1.5,    e=from ≧0 to ≦10,    f=from ≧0 to ≦0.2 and    n=a number which is determined by the valency and frequency of the elements in I other than oxygen,
in such a way that the propene conversion in a single pass is ≧90 mol % and the associated selectivity of acrolein formation is ≧80 mol %.
2. Description of the Background
Acrolein is a reactive monomer which is especially significant as an intermediate, for example in the preparation of acrylic acid by two-stage heterogeneously catalyzed partial gas phase oxidation of propene. Acrylic acid is suitable as such or in the form of its alkyl esters, for example, for preparing polymers which may find use, inter alia, as adhesives or water-absorbent materials.
The preparation of acrolein by the heterogeneously catalyzed partial gas phase oxidation process as described in the preamble of this document is known (cf., for example, DE-A 10351269, DE-A 10350812, DE-A 10344149, DE-A 19948523, DE-A 10313209, DE-A 19948248, DE-A 19855913 and WO 02/24620). Typically, it forms the first stage of a two-stage heterogeneously catalyzed gas phase partial oxidation of propene to acrylic acid. In the first reaction stage, the propene is substantially partially oxidized to acrolein and, in the second reaction stage, the acrolein formed in the first reaction stage is substantially partially oxidized to acrylic acid. It is significant that the industrial embodiment is normally configured in such a way that the acrolein formed in the first reaction stage is not removed, but rather conducted into the second reaction stage as a constituent of the product gas mixture leaving the first reaction stage, optionally supplemented by molecular oxygen and inert gas, and optionally cooled by direct and/or indirect cooling.
The target product of a heterogeneously catalyzed partial gas phase oxidation of propene to acrolein is acrolein.
A problem in all heterogeneously catalyzed gas phase partial oxidations in a fixed catalyst bed is that the reaction gas mixture, as it flows through the fixed catalyst bed, passes through a maximum value (known as the hotspot value).
This maximum value is composed of the external heating of the fixed catalyst bed and the heat of reaction. For reasons of suitability, the temperature of the fixed catalyst bed and the effective temperature of the fixed catalyst bed are therefore also distinguished from each other. In this context, the temperature of the fixed catalyst bed refers to the temperature of the fixed catalyst bed when the partial oxidation process is performed, but in the theoretical absence of a chemical reaction (i.e. without the influence of the heat of reaction). In contrast, effective temperature of the fixed catalyst bed refers to the actual temperature of the fixed catalyst bed taking into account the heat of reaction of the partial oxidation. When the temperature of the fixed catalyst bed is not constant along the fixed catalyst bed (for example in the case of a plurality of temperature zones), the term temperature of the fixed catalyst bed means the (numerical) average of the temperature along the fixed catalyst bed. It is significant in the aforementioned context that the effective temperature of the fixed catalyst bed likewise passes through the hotspot value with the temperature of the reaction gas mixture in the flow direction of the reaction gas mixture.
Disadvantages of the known processes according to the preamble of this document are that, for a given conversion of propene (based on single pass of the reaction gas mixture), the hotspot temperatures of the fixed catalyst bed associated with the catalysts recommended in the prior art, at the required propene hourly space velocities on the fixed catalyst bed, are too high (high hotspot temperatures are normally disadvantageous in that high temperatures firstly accelerate the aging process of the fixed catalyst bed (certain movement processes within the active composition of the catalysts which contribute to aging proceed, for example, more rapidly) and secondly reduce the selectivity of target product formation), which is why the catalysts in the prior art are generally present in the fixed catalyst bed necessarily diluted with inert material-according to specific dilution profiles. However, for a given conversion of propene, the latter limits the possible hourly space velocity on the fixed catalyst bed of propene present in the starting reaction gas mixture.